In order to prepare for a surgical incision in the region of the hip and, in particular, in an operation on the pelvic bone (e.g., in the region of the hip joint) such as when inserting a hip joint cavity, it is necessary for the hip to be registered, i.e., for the spatial position of the hip or pelvic bone to be detected. When registering the hip, the spatial position of the mid-sagittal plane, which is a plane of symmetry of the pelvis, and the pelvic inlet plane or anterior pelvic plane, which is defined by two spinal points and a pubic point, can be ascertained. One spinal point, also commonly referred to as the ASIS (anterior or superior iliac spine), is a characteristic point or anatomical landmark on the pelvic bone and can be easily located on the pelvic bone. The left-side spinal point (lasis) and right-side spinal point (rasis) are indicated in FIG. 9. A pubic point is a characteristic point on the mons pubis of the pelvic bone and is indicated in FIG. 9 by rpubis for the pubic point of the right-side pelvic bone and by lpubis for the pubic point of the left-side pelvic bone.
In order to register the hip, the patient is placed in a supine position and the positions of the spinal and pubic points are palpated by a surgeon. These points can be communicated to navigation software by a pointer provided with markers that can be detected by a camera. Once the corresponding points have been recorded with the patient in the supine position, the patient is placed in a lateral position in order to make a surgical incision in the region of the right-side or left-side pelvic bone, such as, for example, to insert an artificial hip joint cavity, the position of which is defined by the position of the mid-sagittal plane and the anterior pelvic plane as defined by the spinal and pubic points.
If inaccuracies arise while determining the spinal and pubic points, or there is a change in the position of the pelvic bones while turning the patient from the supine to the lateral position, then an inaccuracy may result when inserting an artificial hip joint. This can lead to complications arising for the patient, for example, because the incorrect positioning of the artificial hip joint can cause pain. Similarly, a hip joint cavity can be subject to a high degree of wear due to improper positioning, such that the service life of the implant is reduced.